Sound translating apparatus



Sept- 4, 1956 f M. LTQUGER ETAL 2,761,912

SOUND TRANSLATING APPARATUS Filed May 3l. 1951 2 Sheets-Sheet l Sept. 4, 1956 Filed lay 31. 1951 M. L. TOUGER ET AL SOUND TRANSLATING APPARATUS 2 She'etsSheet 2 #53pm/sf /A/ as.

A2556 ,424,1 ,4Z/fa INVENTORS Alfred H. Kali-ler Maz 1n L. Tanger ATTORNEY United States Patent O 2,161,912 e soUNn rRANsLA'rING APPARATUS Martin L. Touger, Audubon, and Alfred H. Kettler, Collingswood, N. I., assignors, by mesne assignments,

to the United States of America as represented by the Secretary of the Air Force Application May 31, 1951, serial No. 229,144

z claims. (c1. 179-101) The present invention relates to sound translating apparatus, and more particularly to a headset receiver of I spouse with relatively large differences in density of the medium in which they are operated. v y y The dynamic earphone or headset receiver has been found to be the most feasible for `otlering improvements inresponse over other types of earphones in use at the present time. For example, as more particularly explained in applicants copending application, Serial No. 213,022, filled February 27, 1951, the output level of dynamic sound translating instruments is improved by introducing into the vibratory system an acoustic stitiness control chamber of such magnitude as to constitute substantially the sole acoustic control over diaphragm movement. y The results of using this system are that the earphone has a minimum loss in sensitivity with changes in density as between densities found at sea level and 40,000 feet.

y It is a primary object ofthe present invention to provide a sound translating device which will have a substantially uniform frequency response characteristic when operated in uid mediums of widely varying'densities.

It is also an object of the present invention to provide an improved vibratory system for sound translating apparatus used in mediums of widely varying densities.

It is anotherv object of the present invention to provide adynamic headset receiver the vibratory system of which will function to provide substantially constant damping independentof density ofthe Huid medium in which the instrument is operated.

I, Another object of the presentinvention is to provide a transducer which will have substantially uniform response over a wide range of sound frequencies irrespective of density of the medium in which it is operated.

. A furtherobject of the presentinvention is to provide an improved transducer which is simple in construction and highly eicient in use.

In accordance `with the present` invention, an improved vibratory system is provided for a dynamic sound translating device having a structure which forms an acoustically closed cavity on one side of the vibratory member or diaphragm the cavity being of such magnitude as to provide acoustic stiffness control for the vibratory member. A portion of the structure is arranged to provide a cavity on the other side ofthe vibratory member. A plurality of sound transmitting passages are disposed in the structure communicating'the last mentioned cavity withA the medium in which the device is used. Certain lCe ones of the passages are arranged to deline acoustic resistance and inertance components in series in the vibratorysystem while the remainder of the openings dene an acoustic inertance in the system in shunt with the components provided by the certain passages. The remainder passages are arranged to provide an acoustic inertance component which is responsive at relatively lower densities to lower the resistance component of the plurality of passages to high frequency sound waves.

The novel features characteristic of the present invention, as well as additional objects and advantages thereof, will be better understood from the following detailed description when read in connection with the accompanying drawing in which,

Figure l is a front elevation of a headset receiver in accordance with the present invention,

Figure 2 is a sectional view of the receiver shown in Figure l, taken on the line 2--2 of Figure 1,

Figure 3 is an enlarged view of the cover member for the transducer case shown in Figure 2,

Figure 4 is a set of curves showing the high frequency response characteristic of an undamped earphone, taken at pressures corresponding to densities at sea level and 40,000 feet altitudes,

Figure 5 is a set of curves showing the high frequency response characteristic of an earphone provided with conventional damping, taken at pressures corresponding to densities at sea level, 20,000 and 40,000 feet altitudes,

Figure 6 is an equivalent electrical circuit diagram of the acoustical network of the earphone used in obtaining the frequency response curves shown in Figure 5,

Figure 7 is an equivalent electrical circuit diagram of the acoustical network of the earphone shown in Figures l and 2, and

Figure 8 is a set of curves showing the high frequency response of the earphone shown in Figures 1 and 2, taken at pressures corresponding to densities at sea level, 20,000 and 40,000 feet altitudes.

Before describing a preferred embodiment of ,the present invention in detail, it is desired to point out generally the essence of the invention. For the purpose of explanation, a dynamic earphone is employed which is intended for use in a medium of widely varying density, for example, air, the density of which varies inversely with altitude. Under such circumstances, it is found that ordinary dynamic earphones in use today show considerable variation in output level or sensitivity as well as frequency response. In order to maintain substantially uniform output level, it is necessary to employ a vibratory member which is acoustically stiffness controlled, for example, in a manner more particularly explained in applicants above-identified copending application.

lf the earphoue is designed for a sea level resonance of 5000 cycles per second with the vibratory member acoustically stiffness controlled below resonance, and damping provided to atten the frequency response curve to about 6000 cycles per second, then, at 40,000 feet, where the air density is only about onedifth that yat sea level, the earphone is overdamped and the response is reduced to about 2000 cycles per second. Such an earphone may, for example, be designed to employ earphone structure similar to that shown and described in applicants above-mentioned copending applications wherein a substantially acoustically closed back cavity is introduced to control diaphragm movement below the resonant frequency of the vibrating system. Bynproperly proportioning the sound transmitting passages in the casing leading to the cavity in front ofthe diaphragm so as to introduce an inertance component in the acoustical system two resonance peaks are introduced into the frequency response of the earphone to extend theihigh,

frequency range. The response characteristic for such an earphone then will be as shown by the curves in Figure 4 of the drawing, wherein curves A and B represent, respectively, the response taken at pressures equivalent to densities such as at Sea level and 40,000 feet. If, in addition, to introducing the inertance component, damping material is employed over all of the sound transmitting passages, in a manner to dampen the two resonance peaks thereby to effect a desired high frequency response at sea level density, then the response for such an earphone will be as shown by curve A in Figure 5 of the drawing. lf, however, such an earphone is operated at altitudes of 20,000 and 40,000 feet, the frequency response characteristic thereof will be as shown, respectively, by curves B and C in Figure 5. A comparison of these curves shows that the damping introduced for desired response at sea level causes a significant reduction in high frequency response at 40,000 feet. This is an undesirable condition for optimum response where an earphone of the type described is intended for use in a medium of widely varying density.

The acoustical circuit for such an earphone may be represented by an equivalent electrical circuit diagram as shown in Figure 6 of the drawing wherein:

F=the R. M. S. value of the force on the diaphragm,

in dynes;

B=fiux density in the air gap, in gauss;

:length of coil wire in the voice coil, in centimeters;

i=R. M. S. value of the current in the voice coil, in

abamps;

Mf=the effective diaphragm and coil mass, in grams;

A2Sbc=reiiected acoustic stiffness of the chamber behind the diaphragm, in dynes/cm.;

A2Sfc=reected acoustic stiffness of the chamber in front of the diaphragm, in dynes/cm.;

A2La=reflected acoustic inertance provided by the sound transmitting passages;

A2Ra=reected acoustic damping resistance in the earcap openings, in dynes/cm./sec.; and

A2Se=refiected acoustic stiffness of the chamber formed between the earcap and the head of the user, in dynes/ cm.

The acoustic reactive components for the earphone shown in the diagram in Figure 6 vary in impedance in proportion to variations in air density. Consequently, the resonant frequencies are reduced at reduced air densities. Since the acoustic resistance provided by the damping material is independent of density (altitude) and the reactive components have reduced impedance at the reduced air densities, the effect of the acoustic resistance is to produce an overdarnped response at the reduced air densities.

It is desirable, therefor, to provide a damping comportent in the earphone vibratory system which will etfect substantially constant damping independent of altitude. One way of accomplishing this would be to use a damping material the resistance component of which could be varied proportionately in response to changes in density and frequency. But this method is not obtainable with present day materials. However, with the present invention it is possible to achieve such a result by introducing an inertance component in shunt (parallel) with the inertance component containing a resistance compoent, the latter being shown in the acoustical network in Figure 6 of the drawing by the elements AZLB and AzRa. The acoustical network would then be as shown in Figure 7, wherein A2La=retiected acoustic nertance provided by the sound transmitting passages containing no damping resistance.

A preferred embodiment representative of the network shown in Figure 7 is shown in Figures l and 2 of the drawing wherein similar reference characters represent corresponding parts throughout. In this embodiment, the earphone 1 comprises a housing 3 of a design suitable for use in a headset or aviators helmet and a dynamic sound translating unit 5 mounted within the hous-V yoke 11, an annular outer pole piece or plate memberl 13, a permanent magnet core 15 and an inner, cylindrical pole piece 17. The magnet core 15 and the inner pole piece 17 are coaxially mounted and secured by suitable means in cnd-to-end relation. The outer pole piece 13 is provided with a central, circular aperture l19 and is mounted across the open end of the field yoke 11. The magnet core 15 and inner pole piece are concentrically mounted within the yoke 11 with the inner pole piece disposed within the aperture 19 of the outer pole piece 13 and in spaced relation to the outer pole piece so as to provide an annular air gap 21 therebetween.

The diaphragm 9 is made from lightweight material, such as aluminum, and comprises a disc-like member supported at its periphery between a pair of annular spacing rings 23 of non-magnetic material. The voice coil 10 is attached tothe central dome portion 25 of the diaphragm V9. The diaphragm 9 is mounted on the outer pole piece 13 with the voice coil 10 freely disposed within the air gap 21. The diaphragm 9 and voice coil 10 are arranged for simultaneous vibratory movement and' serve to convert electrical signals fed into the voice coil from an external source into corresponding acoustic' waves generated by the diaphragm, in a manner well known in the art.

An annular seal ring 27 of non-magnetic material'isA disposed in.closeiitting relation around the inner pole piece 17. The ring 27l is arranged to extend across the air gap 21 and abut the outer pole piece 13 so that an` entirely closed cavity 29 is provided behind the dia-V phragm 9. An air pressure equalization opening or re stricted passageway 31 is disposed on the inner periphery of the seal ring for connecting the cavity 29 with the uid medium within which the earphone is being operated.l

the earphone is being operated. Although an opening' is providedl as a means for equalizing pressure, other suitable arrangements may be found desirable.

The sound translating unit 5 is mounted in any con` venient manner which will securely hold it inthe housing 3. In order to securely fasten the translating unit in the housing against movement, a spring 33 is disposed between the field yoke 11 and the back of the housing 35.

The housing 3 is also provided with a protective coverA 37 mounted in spaced relation to and in front of the diaphragm 9. A plurality of apertures 39, 41 are provided in the cover 37 for the purpose of transmitting sound waves generated by the diaphragm to the ambient or, when the earphone is Worn by the user thereof, to the cavity provided by the earcap (not shown) surrounding the earphone. The apertures 39, 41 are arranged centrally of the cover with a single, circular aperture 39 at the center and three arcuate-shaped apertures 41 disposed concentrically about and in spaced relation to the center aperture. The outer, arcuate-shaped apertures 41 are covered with damping material 43, such as a wire screen of a mesh suitable to introduce a resistance component into the vibratory system thereby to effectively damp rei sponse in the high frequency range. rlv'he centraluaperf ture 39 is effectively open, that is, there is no damping material used over the opening. However, a dirt filter in the form of a screen 45 may be disposed over the central opening, if desired, but it should have openings of a size as not to introduce a resistance component inta the vibratory system. The size of the central opening 39 is arranged so as to provide negligible effect over the response of the earphone at maximum operating densities and to reduce the resistance component oifered by the damping material 43 disposed over the arcuate aper tures 41 at substantially reduced air densities. ln other words, as shown in the equivalent acoustical circuit diagram of Figure 7, the system includes the combination of an inertance (central aperture 39) shunting a damping resistance (screen 43) which is in series with another acoustic inertance (arcuate apertures 4l).

One particular embodiment which was constructed and successfully operated comprised a dural diaphragm of .001' inch thickness having an effective area of approximately 6 om?, an effective mass of approximately 60 milligrams, and an effective stiffness of approximately 4.5 106 dynes/cm. An aluminum, edge-wound, selfsupporting voice coil having an effective mass of about 60 milligrams was supported from the diaphragm and disposed in an air gap having a magnetic eld of 9,000 gauss. An acoustically closed chamber was provided behind the diaphragm of a design in accordance with the teachings set forth in applicants above-identified copending application and in order to provide acoustic stiffness control for the diaphragm. The volume of the chamber behind the diaphragm was determined to be .6 cc. in order to maintain a ratio of reliected acoustic stiffness to diaphragm stii"'ness of about 35 to 1. The earphone resonant frequency was about 5000 cycles per second. The earphone sensitivity was 24 dynes/cm. output pressure in a 6 cc. coupler for 1 milliwatt of electrical input power.' The cover of the housing was disposed in spaced relation to the diaphragm in a manner to provide an acoustic chamber in front of the diaphragm of approximately 1.8 cc. The cover was made from .018 inch thickness steel and was provided with a central circular aperture of 1A; inch diameter. A set of 3 arcuate openings was disposed in spaced, circumferential relation to the central aperture each of which was provided with an inside radius of 5/32 inch and an outside radius of %,2 inch and with an area of hole opening of .27 square inch. in order to provide suitable damping, a wire screen was made from Lektromesh material having .005 thickness with .002 wide square openings, at a 120 count per inch. This material was disposed over each of the arcuateshaped openings. In addition, a dust proof screen was disposed over the central aperture. The dust proof screen had openings of such a size as to provide negligible acoustic effect over the operating range of the earphone.

Figure 8 of the drawing represents the frequency response curves of this particular earphone, as measured on a 6 cc. American Standards Association coupler taken at densities corresponding to sea level, 20,000 and 40,000 feet altitude pressure. Curves A, B, and C represent, respectively, the earphone response measured at sea level. 20,000 and 40,000 feet pressures. As observed from Fig- 8, comparison of the curves demonstrates the improvement provided by the present invention by showing that the vibratory system of the earphone functions to provide substantially constant damping independent of density of the fluid medium in which the earphone is operated.

From the foregoing, it is apparent that the present nvention provides substantially uniform response over a wide range of sound frequencies irrespective of densities of the medium in which the instrument is operated. Although but a single preferred embodiment of the present invention is illustrated and described herein, it should be obvious to those persons skilled in the art that various said cavity to the medium to provide for pressure equalchanges and modifications are possible within the spiri of the present invention. For example, the size, shape and relationship of the openings may be varied provided the acoustic resistance and inertance components of the damping filter are maintained. For a particular application, the required values of the parameters forming the damping filter which are necessary to obtain maximum high frequency response will be a function of the components provided by the other elements in the vibratory system. Other changes of like character will, no doubt, readily suggest themselves to those skilled in the art. Therefore, it is desired that the particular form of the invention shown and described herein shall be considered as illustrative and not as limiting.

What is claimed is:

l. in a transducer for use in a medium of widely varying densities, a vibratory system comprising means having two pressure-sensitive surfaces adapted to vibrate and to interconvert electrical and acoustical energy, a structure forming an acoustically sealed cavity in conjunction with one of said surfaces, said cavity being dimensioned to provide acoustic stiffness control for said vibratory means, a restricted passageway for connecting ization, a portion of said structure forming a cavity in conjunction with the other of said surfaces, at least one passage in said structure defining acoustical resistance and inertance components in series in said vibratory system and establishing communication between said last mentioned cavity and the medium, and at least one other passage in said structure defining an inertance component in said vibratory system in shunt with the components of said first mentioned passage, the size of said last mentioned passage being so selected as to reduce the resistive component of said first mentioned passage at reduced densities of the medium whereby a constant damping is introduced into the vibratory system independent of density.

2. ln a transducer for use in a medium of widely varying density, a vibratory system for said transducer comprising means having two pressure-sensitive surfaces and being adapted to vibrate and to interconvert electrical and acoustical energy, a structure forming an acoustically sealed cavity on one side of said vibratory means, said cavity being dimensioned to provide acoustic stiffness control for said means, a restricted passageway for connecting said cavity to the medium to provide for pressure equalization, a portion of said structure forming another cavity on the opposite side of said vibratory means, said structure being provided with a plurality of sound transmitting passages communicating said last mentioned cavity with the medium, acoustic damping means associated with certain ones of said passages," said certain ones of said passages together with said damping means defining acoustic inertance and resistance components respectively in series in the vibratory system for controlling the response of said transducer over the operating range thereof at the maximum density of the medium, the remainder of said passages delining an inertance component in the vibratory system in parallel with the components of said certain passages, said remainder passages being arranged to provide negligible effect over the response of said transducer at maximum densities of said medium and to reduce the resistance component of said damping means at substantially reduced air densities.

References Cited in the le of this patent UNITED STATES PATENTS 1,951,531 Ringel Mar. 30, 1934 2,238,741 Lauffer ,Apr. 15, 1941 2,252,846 Giannini et al. Aug. 19, 1941 2,515,031 Black July 11, 1950 2,517,138 Seabert Aug. 1, 1950 2,549,963 De Boer et al Apr. 24, 1951 

